1) Field of the Invention
The present invention relates to a variable optical attenuator and an optical filter that includes the variable optical attenuator.
2) Description of the Related Art
In recent years, large-capacity high-speed optical communications have been promoted. Accordingly, bandwidth extension is in increasing demand. To respond such demands, various optical devices have been developed and introduced. For example, a tunable filter technology such as a wavelength selection switch and a dynamic gain equalizer for Wavelength Division Multiplexing (WDM) in optical communications is an important technology in a dynamic optical network.
To achieve a tunable filter with high resolution, a free-space optical system has been suggested, which includes a lens, a diffraction grating, and an optical device with pixelized optical elements formed in an array. In such a free-space optical system, a light beam incident from outside via an optical fiber on an incident side (hereinafter, “incident-side optical fiber”) is branched to a predetermined optical element pixel of the optical device, and the light branched is reflected from the optical element pixel. The light reflected is then multiplexed to enter (be coupled to) an optical fiber on an emission side (hereinafter, “emission-side optical fiber”), and is then emitted to outside.
In the free-space optical system, an amount of light entering the emission-side optical fiber can be adjusted by the optical device. In this example, the optical device corresponds to a variable optical attenuator that adjusts an amount of attenuation of incident light so as to adjust the amount of light. For such variable optical attenuator, which is formed in an array, for example, a technology of micro-electro-mechanical systems (MEMS) has been developed and applied, such as MEMS of an analog tilt mirror scheme or of a diffraction type, and a digital micromirror device (DMD).
There is another type of a variable optical attenuator that uses a liquid crystal technology besides the technology of MEMS. Such variable optical attenuator is one of important options for achieving a tunable device that meets the demands for the dynamic optical network, because of such advantages as large birefringence of liquid crystal, easy integration with a standard complementary metal oxide semiconductor (CMOS) circuit, and a high degree of maturity of liquid crystal technology derived from a technology for a liquid-crystal display.
Unlike the variable optical attenuator based on MEMS technology, the variable optical attenuator based on the liquid crystal technology does not require a movable component. In the MEMS, due to the movable component, such as a mirror, internal or external disturbances, such as friction, stiction (a phenomenon in which a movable component is attached to another component), vibration, and electrification are caused. Such troubles may be a factor of a failure in the variable optical attenuator. Therefore, there is a high possibility of failures in such variable optical attenuator based on the MEMS. On the other hand, in the variable optical attenuator based on the liquid crystal technology, the movable component is not required. Therefore, there is no possibility of failures due to such problems, and thus, the variable optical attenuator based on the liquid crystal technology has higher reliability.
Moreover, the variable optical attenuator based on the MEMS generally requires a higher driving voltage than that in the variable optical attenuator based on the liquid crystal technology. Furthermore, because of the movable component, a manufacturing process tends to be complex in the variable optical attenuator based on the MEMS. Compare to the variable optical attenuator based on the MEMS, the variable optical attenuator based on the liquid crystal technology requires a lower driving voltage and lower manufacturing cost. In addition, the variable optical attenuator based on the liquid crystal technology can be formed in a reduced size, and is suitable for integration with the standard CMOS circuit.
FIG. 28 is a schematic of a conventional variable optical attenuator that includes a liquid crystal element (for example, refer to Japanese Patent Application Laid-Open Publication No. 2001-13477). As shown in FIG. 28, an incident light L271 emitted through a fiber collimator 271 enters a polarized beam splitter 272 on an incident-side. The polarized beam splitter 272 is a polarizing and separating unit, and separates the incident light L271 into a p-polarized light beam and an s-polarized light beam. Both the p-polarized light beam and s-polarized light beam enter a liquid crystal element 276, which is a polarization rotator element.
The liquid crystal element 276 is formed with two pieces of 45-degree-twisted nematic liquid crystal cells (hereinafter, simply “liquid crystal cells”) 274 and 275 that are laminated, and that are formed such that each of the liquid crystal cells 274 and 275 adjusts an applied voltage independently. In a process where the p-polarized light beam and the s-polarized light beam entering the liquid crystal element 276 pass through the liquid crystal cells 274 and 275, the amount of light emitted from the liquid crystal element 276 is adjusted.
The light beams emitted from the liquid crystal element 276 further enter a polarized beam splitter 273 on an emission side. The polarized beam splitter 273 is a polarized-light combining unit, in which the light beams are multiplexed. The light beams multiplexed are emitted as emitted light beams L272 and L273 from side surfaces 273a and 273b of the polarized beam splitter 273 via fiber collimators 278 and 279, respectively.
In the conventional variable optical attenuator described above, the polarized beam splitter 272 and the polarized beam splitter 273 are required to be provided separately on the incident side and the emission side. Therefore, the number of components increases, thereby making it difficult to reduce the size of the variable optical attenuator. Furthermore, such structure increases the manufacturing cost, and makes the manufacturing process complex.
In addition, in the process in which the p-polarized light beam and the s-polarized light beam pass through the polarized beam splitter 272 and the polarized beam splitter 273, a difference in optical path length occurs between the polarized beams. As a result, polarization mode dispersion (PMD) of the variable optical attenuator is deteriorated.
Moreover, if a polarization axis of each of the polarized beam splitter 272 and the polarized beam splitter 273 is not arranged at an appropriate angle with respect to a liquid crystal director of the liquid crystal element 276, polarization dependent loss (PDL) occurs.